Fin field-effect transistors (FinFETs) are widely used in integrated circuits for their increased gate widths over planar transistors. A major advantageous feature of the FinFETs is that they use small chip areas. To maximize this benefit, when used in high-density devices, for example, static random access memory (SRAM) arrays, the FinFETs were often designed as single-fin FinFETs.
The SRAMs with single-fin FinFETs, however, often suffer from low cell ratio problems such as low beta ratios. The beta ratios are the ratios of the drive currents of pull-down transistors to the drive currents of the respective pass-gate transistors. The beta ratios are important for the cell stability. Generally, the beta ratios are preferably greater than 1. In the formation of high-density SRAM arrays, however, this requirement results in process difficulties. For example, it is difficult to meet this requirement while at the same time keep the cell size small.